Optical disk recording devices include an objective lens which focuses a laser light beam onto an optical disk while the disk is rotating. Focus and tracking actuators are typically used in optical disk recording devices to control the position of the objective lens relative to the rotating disk. Both focus and tracking of optical disks must continuously be maintained while the disks are recording. Focus is controlled by adjusting the distance between the objective lens and the disk surface. Tracking is controlled by correcting the radial position of the objective lens so that a single data track is read or written at a time. The motions of the actuator in the focus and tracking directions are conventionally controlled by a servo system.
Alignment of the optical axis of the objective lens to the optical disk must be maintained accurately for the system to function properly. Any tilt of the disk surface relative to the optical axis of the objective lens reduces overall system performance. A significant contributor to this undesirable tilt is the non-flatness, or warp, of the disk itself, which is hereinafter referred to as "disk warp." Disk warp is typically caused by shrinkage in the manufacturing process, and other causes such as material nonuniformity or imperfections. Disk warp generally results in tilt of the disk surface along each of two orthogonal directions. Radial tilt is rotation of the disk surface about an axis along the radius of the disk, and tangential tilt is rotation of the disk surface about an axis perpendicular to the radius of the disk within the nominal plane of the disk.
Typical prior art optical recording actuators cannot compensate for disk warp. The presence of disk warp limits the overall system performance, reduces recording quality, and requires higher laser power. In addition, the use of higher numerical aperture lenses is limited largely by the high tilt sensitivity associated with high numerical aperture lenses, which limits the recording information density on the optical disk.